1. Field of the Invention
The invention relates to a method for purifying aqueous residues from exhausted metallizing baths, such as baths customarily used in the production of printed circuits for forming predetermined circuitry of copper or other metallic conductors on appropriate substrates, and to the use of this method for working up exhausted baths for the production of printed circuits by currentless deposition of copper.
2. Description of the Prior Art
Modern industrial metallizing baths, such as are widely used for depositing copper and other heavy metals by electrolytic methods or methods which operate without external current, especially for the production of electrical or electronic components, for example printed circuits, even in the exhausted state contain substantial amounts of components which have not been consumed. Ecological and economic reasons require that such baths be worked up in such a way that harmful emissions are prevented, using the least possible amount of energy and at the lowest possible cost.
This problem is particularly serious because, when the exhausted baths are worked up, the amino compounds contained in such baths as the complex-forming agent, precisely because of their pronounced action in preventing the precipitation of cationic constituents, oppose the main object of working up, that is to say the recovery of an effluent which is virtually free from heavy metal ions.
Such complex-forming agents containing amino groups, such as the water-soluble salts of ethylenediaminetetraacetic acid (EDTA), are in themselves virtually acceptable from the physiological point of view and are used in many industries and even for therapeutic purposes (accelerated excretion of heavy metals from the body). Many of these complex-forming agents and specifically EDTA are, however, chemically so stable that they do not decompose with the customary methods and can be destroyed only by very strong oxidizing agents or by thermal decomposition. However, this is again ecologically and/or economically disadvantageous.
On the other hand, the industrial working up of solutions or effluent concentrates of various origins which contain heavy metal ions is usually based on the precipitation of hydroxides from the mixed solutions and a treatment of this type is, of course, not possible when one of the concentrates still contains complex-forming agents of said type, especially EDTA.
Conventional processing is, thus, virtually not economically practical and the solubility of heavy metal ions bonded as a complex with such complex-forming agents or chelating agents cannot be reduced by the conventional methods of working up such baths. It was necessary to reduce the proportion of complex-forming agents in the aqueous phase virtually to zero when working up exhausted metallizing baths, in order to keep within the permissible limiting concentrations for heavy metal ions in the effluent, and this cannot be achieved in an economic manner according to the state of the art with ecologically admissible agents. There are also limits on the method which has usually been necessary hitherto, and which is ecologically unsatisfactory, which comprises extensive dilution of the effluent from such a work-up in order to keep within the permissible concentrations for heavy metals.
A need continues to exist for a technique of treating exhausted metallizing baths to produce an ecologically acceptable effluent economically.